Morphogenesis of stem gall tissues induced by larvae of two cecidomyiid species (Diptera: Cecidomyiidae) on Suaeda monoica (Chenopodiaceae)

Izeniola obesula Dorchin and Stefaniola defoliata Dorchin (Diptera: Cecidomyiidae: Lasiopterini) are monophagous gall midges each inducing a unique kind of gall on stems of the salt marsh plant Suaeda monoica Gmelin (Chenopodiaceae). The morphogenesis of these two types of galls was studied in relation to the life history of the midges as observed both in the field and the laboratory. Izeniola obesula larvae penetrate the pith parenchyma through the growing shoot apex, causing intensive cell proliferation and inducing differentiation of novel vascular tissues and a sclerenchyma sheath around their chambers. Vascular differentiation in this gall originates from the larval chamber, a phenomenon attributed to local stimulation by the larva. It is suggested that the sclerenchyma layer in these galls is also induced by insect activity. Stefaniola defoliata larvae penetrate the stem laterally and reside inside the primary phloem, causing proliferation of phloem parenchyma, and are later encapsulated by secondary xylem tissue. Both galls are associated with a symbiotic fungus that grows along the inner walls of the larval chambers. The possible hormonal mechanisms controlling morphogenesis of the galls are discussed.